1. General Field
My invention is in the field of fiber optics, and particularly relates to connectors for fiber-optic circuits. Though my invention has broad general application for such connectors, it has in particular a specialized application in the area of "penetrators" or feed-throughs across high-pressure barriers.
2. Prior Art
Conventional fiber-optic connectors consist of accurately polished tips on the ends of the two fibers to be connected, flat and perpendicular to the axes of the respective fibers, and elaborate hardware for presenting the two flat tips in precise conaxial alignment for mutual abutment. It is a characteristic of such connectors that the two fiber end-faces must seat accurately against each other each time the connector is plugged together.
Ideally the separation of the faces is zero. If they are separated slightly, power transmission between them falls off, following an inverse-square law. On the other hand, if the faces are moved together too forcefully they can grind each other.
Moreover, as will be discussed in greater detail below, accuracy is also essential in aligning the fibers laterally.
Aside from the apparent mechanical design constraints and resulting costs imposed by these accurate-seating requirements, such connection geometries invite malfunction of the optical circuit--and also an aggravated possibility of damage by scratching--if even a very small particle of foreign matter is trapped between the two faces of the connector.
To my knowledge no connector design avoiding this accurate-seating requirement and suitable for practical production-quantity application has been placed in general use or even proposed heretofore.
In recent years there has developed a new kind of optical element, particularly suited for fiber-optic circuits: the "graded-index rod," sometimes abbreviated "GRIN rod." Such rods, which are characteristically many times larger in diameter than typical fibers, have systematically varied index of refraction--generally a maximum value along the central axis of the rod, with refractive index gradually decreasing (usually as a nearly parabolic function) with radial distance from the axis. The pattern of refractive index versus radius is cylindrically symmetrical to a high degree of accuracy. GRIN rods have a remarkable property: within such a rod, optical rays diverging from the axis are progressively deflected (refracted) toward the axis. Those rays which leave the axis within a maximum-apex-angle cone, depending on the precise function followed by the refractive index and also on the total outside diameter of the rod, eventually become parallel to the axis--and then, under continuing influence of the gradation of refractive index, continue to be deflected toward the axis and finally cross the axis.
Because of the cylindrical symmetry of the index gradation, all rays diverging from a particular point on the axis reconverge on the axis at a common point regardless of the initial angles (about the axis) at which they are oriented. Moreover, the refractive-index variation is such that the reconvergence point is independent of initial angle of divergence from the axis. The path of each ray is nearly sinusoidal, crossing and recrossing the rod axis with progress down the rod, the distance between nodes depending only on the wavelength of the light. It is common to refer to a GRIN rod whose length is exactly equal to the distance between adjacent nodes, which is to say half of the "wavelength" of the sinusoid, as a "half-pitch rod."
Such a rod refocuses light diverging from a point on its axis at one end to the corresponding point on the axis at its other end. Moreover, such a half-pitch rod refocuses light from any point at one end to the corresponding point on the other end of the rod--with an inversion relative to the axis. In other words, a half-pitch GRIN rod is an imaging device, which inverts the image. Such rods have received wide use in imaging applications, and some limited use in pressure-hull penetrators as discussed immediately below, but to my knowledge no utilization relating to fiber-optic connectors as such, heretofore.
Fiber-optic systems have considerable appeal for signal transmission between the modules of undersea rescue or exploration vehicles, for use at extreme depths and extremely high pressures. To avoid failure of such vehicles under the pressures involved, they are sometimes constructed in the form of a plurality of spheres, with one or more carrying human divers and the remainder carrying equipment. Operational monitoring and control signals between these spherical modules can be carried by optical fibers, but the penetration of the spherical hulls by the fibers must be effected in such a way as to avoid forming a stress point in a hull, or of inducing even a small leak. Along these lines, physically penetrating a spherical hull with a small-diameter optical fiber can produce some awkward problems.
For example, U.S. Pat. No. 3,825,320, which issued July 23, 1974 to John T. Redfern, discloses two variations of an optical penetrator for undersea use. One of these makes use of a cylindrical half-pitch GRIN rod (31a in FIG. 2 of that patent), held by epoxy in a cylindrical bore, and the other uses a tapered optic-fiber bundle, within a tapered plug (element 29 in FIG. 1 of that patent).
The cylindrical-rod design relies on the epoxy to prevent the rod from being rammed into the interior of the vessel by the pressure differential. The tapered-core design seeks to minimize that risk by the self-sealing characteristic of a conical frustum pressured at its base; but by converting the axial force on the plug and fiber bundle to radial force on the hull this design applies a splitting force to the hull. The tapered plug has the potential for acting as a wedge to rupture the hull. The fundamental weakness in such a design relates from the use of small-diameter fittings actually physically penetrating the hull, rather than large-diameter, plate-shaped structures applied parallel to and thus reinforcing the hull. But a GRIN rod such as that in the referenced patent cannot (at least in the present state of the art) be made in the form of a large-diameter plate-shaped element.
My invention in its basic embodiment avoids the accurate-seating requirement of the fiber-optic connector art, and in another embodiment provides a deep-undersea-hull optical penetrator design which is naturally compatible with large-diameter reinforcing-plate geometries.